In-band call signalling systems use the same channel for carrying speech and for carrying call signalling messages. In early telephony systems the call signalling messages, consisting of dial pulses, preceded the appearance of voice signals on the channel. Later, dual tone multifrequency signalling came into use. These DTMF signals consisted of pairs of sinusoids lying within the frequency range of ordinary speech. This was possible because the call signaling tones would normally be permitted on only a portion of the telephone channel, usually before the complete end-to-end talking path was established, and so there was little likelihood that any speech components would be present while the DTMF signals were being detected. In the United States, DTMF signalling usually employs one tone in the high frequency group 1209, 1336,1477 and 1633 Hz and one tone from the lower frequency group 697, 770, 852 and 941 Hz.
As telephony has progressed and as digital telephone systems have become more prevalent, it has become more common for both the speech and the call signalling information originating at a telephone set to be digitally encoded either at the telephone set or at a line concentrator serving a number of telephone sets. With both speech signals and DTMF signals being encoded at the same point, the subsequent digital bit stream looks no different when carrying voice, voice and signalling tones or only signalling tones. It has thus become important to be able to more definitely distinguish call signals from speech components. Moreover, it is now common for tone signalling to be employed even after the complete speech path has been set up for such enhanced applications as controlling voice mail disposition, banking by telephone, etc.
Many techniques are known for detecting sinusoids in general and DTMF signals in particular. One sophisticated arrangement, described in Oppenheim and Schafer, Digital Signal Processing, Prentice-Hall, Englewood Cliffs, N.J. 1975, employs a discrete Fourier transform known as Goertzel's algorithm to detect the presence of sinusoidal signals. Other approaches, discussed in U.S. Pat. No. 5,119,322 issued Jun. 2, 1992 propose to detect the DTMF tones by convolving the input signal with sampled values of a reference signal or by repetitively applying Goertzel's algorithm either at each of the DTMF frequencies or in coarse and fine mode. One implementation of Goertzel's algorithm employing digital signal processors is disclosed in an article entitled "Add DTMF Generation and Decoding to DSP-.mu.P Designs" published in Digital Signal Processing Applications with the TMS 320 Family, Texas Instruments, 1989, vol. 1, p. 454 et seq. Basic spectral estimation techniques are disclosed in Digital Spectral Analysis, Marple, S. L., Prentice Hall 1987, pp. 64. While the importance of digitally detecting DTMF tones quickly and accurately has thus been recognized, more efficient techniques are desirable in such advanced digital applications as ISDN.